1. Field of the Invention
The present invention relates to the removal of color imparting impurities from purified wet process phosphoric acid. More specifically, the present invention relates to a decolorization process having improved efficiency from a cost and raw materials standpoint.
2. Description of the Prior Art
Wet process phosphoric acid is typically prepared by digesting mined phosphate ore with a strong acid, such as sulfuric acid followed by filtering and removing of gypsum crystals. The resulting crude liquor typically has a color ranging from greenish-yellow to dark brown. The colorization is thought to be caused by humic acid compounds, i.e., aromatic carbon compounds of high molecular weight, which when oxidized form highly colored quinones. These carbon-containing impurities may originate either from the mineral deposit itself and/or from additives introduced or incorporated during physical treatment of the phosphate rock, such as anti-foaming agents. In order to obtain a technical or food grade acid, the crude acid typically is purified in a continuous liquid-liquid extraction and re-extraction procedure which is well-known to those skilled in the art. See for example U.S. Pat. Nos. 4,349,519 and 4,225,568. The organic carbon content of the crude re-extracted liquor (determined by the amount of CO.sub.2 obtained by oxidation of the solution) typically varies between about 0.01 to 0.1% by weight of the solution.
While the presence of color imparting organic impurities is not undesirable in all possible uses of the wet process acid, in cases such as preparation of either technical or food-grade acid, the presence of color imparting organic impurities is highly undesirable. For such applications the acid should have a color value (APHA color standard) of 30 or less.
A number of methods have been developed in the prior art for removing these color imparting organic compounds. Of these teachings, a number have suggested treating the purified wet process acid with a strong chemical oxidant, such as hydrogen peroxide, as well as by treatment with activated carbon in order to remove the organic impurities. See, for example, German Pat. No. 2,813,755; German Pat. No. 3,131,847; Japanese Pat. No. 57-61606 and U.S. Pat. No. 4,330,516.
In particular, U.S. Pat. No. 4,330,516 to Winand discloses a process for the decolorization of wet process acid. After performing a number of processing steps which are well known and used in this art including extraction and re-extraction, Winand performs a final decolorizing treatment comprising adding an oxidizing agent while heating the acid to an elevated temperature of at least 110.degree. C., preferably at least 130.degree. C., and subsequently adding powdered activated carbon directly to the acid in order to absorb organic impurities. Winand discloses the use of strong chemical oxidants such as chlorates and hydrogen peroxide.
Unfortunately, the processes disclosed by Winand are subject to several serious disadvantages. First, when using chlorates such as chloric acid and sodium chlorate, undesirable impurities (such as sodium and chlorine ions) are added to the acid and must be removed. Additional ion impurity removal steps add significantly to the manufacturing costs.
While the use of hydrogen peroxide as an oxidant does not contribute any undesirable ion impurities, the hydrogen peroxide constitutes an expensive raw material. Similarly, the powdered carbon absorbent also constitutes an expensive raw material which in the Winard process can only be effectively used once, and then must be filtered (an additional processing step) and discarded or regenerated.
Thus, it is an important object of the present invention to provide a more economical process for the decolorization of wet process acid utilizing significantly smaller amounts of both hydrogen peroxide and activated carbon.